A circuit integrated in a chip of semiconductor material is typically supplied by an external supply voltage applied to corresponding terminals of the circuit. In some cases, the integrated circuit includes an analog unit which requires an extremely precise supply voltage with a very high (theoretically infinite) power-supply noise rejection (or PSNR). For example, an integrated circuit may be considered, in which there is a microprocessor and a phase-locked loop (or PLL) which generates an internal clock signal, derived from a corresponding external clock signal, for the microprocessor. The PLL includes a voltage-controlled oscillator (or VCO) which produces a signal having a frequency which depends on a control signal (a voltage) or, more precisely, on a difference between a supply voltage of the oscillator and the control signal.
A problem of this integrated circuit is that the external supply voltage (from which the supply voltage of the oscillator is derived) is subject to interference which is generated, for example, by digital circuits of the microprocessor (for example, during the switching of electronic switches) and which modifies the working frequency of the oscillator. In the example, this generates a skew between the external clock signal and the internal clock signal, with a consequent jitter defined as the difference, in absolute value, between the maximum advance skew and the maximum delay skew.
A known approach includes the use of a separate external supply voltage for the analog unit; however, this requires two additional supply terminals and considerably complicates a system in which the integrated circuit is connected. Another known approach provides for the use of a reference unit (for example, of the bandgap-based type) which is formed in the integrated circuit and can provide a supply voltage of very precise value, irrespective of interference and temperature variations. However, this reference unit is very complex (since it is typically formed by many transistors) and thus occupies a considerable space on the chip in which the integrated circuit is formed.